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Impact of blanching pretreatment on the drying rate and energy consumption during far-infrared drying of Paprika (Capsicum annuum L.)

Impact of blanching pretreatment on the drying rate and energy consumption during far-infrared... We incorporated a superheated steam blanching pretreatment step into a paprika drying process and compared the far-infrared (FIR) drying rates, hardness of the sample surfaces, cell membrane stabilities, and energy consumption of blanched and non-blanched paprika. The average drying rate of blanched paprika samples during FIR drying was higher than that of non-blanched samples. The hardness and cell membrane stability of dried blanched samples were lower than those of non-blanched samples. We estimated that the softening of the sample surfaces and injury to the cell membranes caused the drying rate to increase. The total energy consumption of the FIR drying of paprika was reduced by approximately 30% by introducing the blanching pretreatment. These findings contribute to the development of environmentally friendly FIR drying techniques for paprika. Key words: blanching; cell membrane stability, energy consumption; far-infrared drying; hardness. changes in plant materials that affect the quality of final products, Introduction such as excessive shrinkage, discoloration, oxidation of functional Dried paprika powder is commonly used throughout the world. ingredients, and severe deterioration of nutritional and sensorial Paprika powder has features such as a high nutritional content properties. Far-infrared (FIR) drying can be used as an alternative (including L-ascorbic acid and polyphenol) and good clear colour; to hot-air drying. FIR drying has remarkable advantages including therefore, it is widely used as a spice for cooking. Previous stud- a shorter drying time and improved energy efficiency ( Hebbar et al., ies have reported the drying characteristics and quality changes of 2004). Okamoto et  al. (2012) reported that the energy consump- paprika during the drying process in order to prepare high-quality tion of FIR drying for Japanese mustard spinach leaves was approxi- dried paprika (Topuz et  al., 2009; Guerrero et  al., 2010; Ramesh mately 17% lower than that of hot-air drying. Development of novel et al., 2001). techniques for more environmentally friendly drying methods would Hot-air drying is a simple method commonly used for fruits and contribute to CO emission abatement in the processing of agricul- vegetables (Bazyma et al., 2006). Some disadvantages of hot-air dry- tural products. Increased use of more environmentally friendly dry- ing regarding the drying process parameters include low energy effi - ing techniques over conventional techniques would have advantages ciency and the lengthy drying times required during the falling rate not only towards CO emission abatement but also reduced running period (Drouzas et al., 1999; Bazyma et al., 2006). Traditional dry- costs and improved productivity. ing methods, including hot-air drying, can cause many undesirable © The Author(s) 2018. Published by Oxford University Press on behalf of Zhejiang University Press. This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact journals.permissions@oup.com Downloaded from https://academic.oup.com/fqs/article-abstract/2/2/97/4955764 by Ed 'DeepDyve' Gillespie user on 26 June 2018 98 T. Orikasa et al., 2018, Vol. 2, No. 2 Increasing the drying rate would help reduce energy consumption were cooled immediately in ice water (0°C for 150  s), and then during the drying process. As expected, we found that the blanching excess water on the sample surface was absorbed by paper towel. treatment could enhance the drying rate. Generally, blanching treat- After cooling, the blanched samples were dried using a FIR drying ment can inhibit various undesirable enzymatic reactions and thus chamber. may be required to improve the final quality of the processed prod - uct (Nilnakara et  al., 2009). In addition, blanching pretreatments FIR drying treatment naturally enhance the drying rates of kiwifruit (Yoshida et al., 2014), The drying experiments were conducted in a FIR drying apparatus spinach (Hirota et al., 2000), and cabbage (Nilnakara et al., 2009; as illustrated in Figure  1. The system consists of a drying chamber Watanabe et  al., 2014). Watanabe et  al. (2015) evaluated quality (DKM600, Yamato, Japan) with two FIR heating panels (TE Super changes in paprika slices during an FIR drying process; however, aguri-heater, Toyo Kosan, Japan) connected to 100 V AC power the effects of blanching pretreatment on the drying rate of paprika source. The air velocity during drying was 0.3 m/s, which was meas- samples have not been reported. Therefore, the first objective of this ured by an anemometer (405-V1, Testo, Japan). Six samples were study is to quantitatively evaluate the effects of blanching pretreat- placed skin side down on the steel net at a height of 26 cm in the ment on the drying rate of blanched paprika samples. drying chamber. The sample was considered to have reached its Several reasons why blanching pretreatment accelerates drying equilibrium moisture content when the mass change during 1.0 h of have been proposed. These reasons are as follows: the water perme- drying was less than 0.01 g, and once equilibrium was reached, the ability of the surface is enhanced due to decreased hardening of the drying test was terminated. The mass of the sample during drying sample surface (Orikasa et  al., 2008), internal resistance to mois- was measured at 1.0  h intervals by retrieving the sample from the ture diffusion is reduced by changes in the microstructure due to drying chamber and weighing it using a digital balance (GF-3000, physical damage to the sample (Watanabe et al., 2014), and internal A&D, Japan). After the measurement, the sample was returned to its water permeability is improved due to damage to the cell membranes original position, and the drying experiment was continued. by heat stress (Ando et  al., 2016). These factors interact in a com- plicated manner to enhance the drying rate. However, these previ- Modelling of moisture content changes ous studies have not evaluated the effects and mechanism in detail. Many studies have demonstrated that the changes in the moisture Therefore, the second objective of this study is to evaluate the effects content of agricultural products during the falling rate period of of the blanching treatment that are responsible for the enhanced dry- their drying processes can be predicted by the following exponential ing rate during FIR drying of paprika. This was achieved by meas- model (Kashaninejad et  al., 2007; Srikiatdena and Roberts, 2008; uring the sample surface hardness after blanching and during the Orikasa et al., 2014): drying process, and cell membrane stability (CMS), which was used MM − as an indicator of physical damage of samples. MR = =− exp( kt) (1) MM − 0 e In addition, Ando et  al. (2016) mentioned the importance of investigating the effects of increased drying rate due to blanch- ing treatment on the total energy consumption of the drying pro- where M is the moisture content (d.b.), M is the equilibrium mois- cess. Even if energy consumption during the drying process can be ture content (d.b.), M is the initial moisture content (d.b.), and k is reduced by introducing blanching pretreatment to the paprika dry- the proportional constant (called the drying rate constant) in the first −1 ing process, total energy consumption might be increased due to the drying falling rate period (h ). The M in this study was regarded as energy consumption of the blanching process, and this additional the final moisture content at the end of the drying period [0.0111 step might not contribute to overall CO emission abatement of (d.b.) for the sample blanched for 0 s, 0.1939 (d.b.) for the sample the total process. Therefore, the third objective of this study is to blanched for 103 s, 0.2030 (d.b.) for the sample blanched for 130 s, evaluate the effect of increase of drying rate by the incorporation of and 0.0050 (d.b.) for the sample blanched for 186 s]. The drying blanching treatment on the total energy consumption of the dried constant k was estimated by fitting the obtained moisture content paprika production process. data to Equation (1) using the least squares method. To evaluate the goodness of the fit, the root mean squared error (RMSE) and the determination coefficient ( R ) were calculated. Materials and Methods Materials Paprika (Capsicum annuum L.) was purchased from a local market and stored in a refrigerator at 4°C before measurements. The pap- rika was analysed within 1 week. Once the stalk and seeds were removed, the paprika was divided into eight pieces using a kitchen knife. The mass of each piece was approximately 15  g. The initial moisture content of these samples was 11.35  ±  0.004 [kg-water/ kg-dry solid (d.b.)] as determined using the oven drying method reported by Topuz et al. (2009). Blanching treatment The blanching treatments were conducted using a superheated steam oven (AX-PX-W, SHARP, Japan) at 200°C in a chamber in preheat- ing mode. Six samples were placed skin side down on the steel net in the steam oven chamber. The samples were heated for 0, 106, Figure 1. Schematic diagram of the far-infrared (IFR) drying apparatus. 130, and 186 s (Watanabe et  al., 2015). After heating, the samples Downloaded from https://academic.oup.com/fqs/article-abstract/2/2/97/4955764 by Ed 'DeepDyve' Gillespie user on 26 June 2018 Drying Rate and Energy Consumption During FIR Drying of Paprika, 2018, Vol. 2, No. 2 99 Hardness of the sample surface QP =× ()tW () −W (5) 0e Punching tests were performed to evaluate changes in sample hard- where Q is the energy consumption per 1 kg of water removed via ness after blanching treatment and during the drying process. The the drying process (kJ/kg), P is the electrical power supplied during tests were performed using a food hardness meter (RE2-3305S, the blanching and drying processes (kW), t is the treatment time (h), YAMADEN, Japan) with a 20 N load cell and a flat-ended cylindrical W is the initial mass of the sample (kg), and W is the mass of the probe 3 mm in diameter. The displacement speed was 10 mm/s. The 0 e dried sample at a moisture content of 0.1 (d.b.) (kg). The energy sample was placed on a polyacetal resin flat plate in a food hardness consumptions of the blanching and drying processes were measured meter. In this study, the average fracture load at the centre point of using a power meter (CW240, Yokogawa Meters & Instruments six samples was defined as the hardness of the sample. The blanched Corporation, Japan). The power meter was connected to the power samples were equilibrated at room temperature for 10 min before the source, and the drying devices (i.e., the steam oven and drying cham- hardness measurements because the effect of increased turgor pressure ber) were connected to the power meter. The apparent electric energy in the cells during the blanching process on the hardness of the sam- consumptions during operation of the drying devices were recorded ples (Tamura 1995) must be excluded. The hardness of the samples by the power meter in kWh and were assumed to be the energy con- during the drying process was measured when the samples reached sumptions of the respective processes. a moisture content of 8.0 (d.b.). The measurements for each sample under each set of conditions were conducted in triplicate. Statistics Quantitative data are presented as the means ± the SE. For the stat- Cell membrane stability istical analyses, Tukey–Kramer tests with a significance level of The CMS of fresh and blanched paprika samples was determined P  <  0.05 were applied by using statistics software (BellCurve for by electrical impedance spectroscopy (EIS) as described by Wu et al. Excel, ver. 2.13, Social Survey Research Information Co., Ltd). (2008). The impedance [impedance magnitude |Z| (Ω) and phase dif- ference θ (rad)] of the samples was measured using an LCR tester Results and Discussion with two parallel electrodes spaced 10  mm apart at 50 frequency points (logarithmic frequency intervals) over the frequency range of Drying model and drying rate 42 Hz to 5 MHz at a measuring voltage of 1.0 V (frequency accur- Previous studies have reported that changes in the moisture content acy: < ±0.005%; measurable impedance range: 10.00 mΩ to 200.00 of fruits and vegetables during drying processes that removed more MΩ; HIOKI, 3532-50, Japan), and the values were automatically than approximately 0.1 of the moisture content ratio could be pre- recorded by a computer for analysis. The resistance R and reactance dicted by the exponential model (Ayensu 1997; Sacilik and Elicin, X were calculated from the following equations: 2006; Doymaz, 2007), and the model showed that drying primar- ily occurred in the first falling rate period. To evaluate the drying RZ = cosθ (2) periods in this study, we fitted the moisture content data from the initial measurement to a moisture content ratio of 0.1 to Equation (1). The solid lines in Figure 2 represent the results calculated from XZ = sinθ (3) Equation (1). The calculated results agreed with the measured results (RMSE = 0.260–0.276; R  = 0.998–0.999). Within the drying time The relationships between the real part of the impedance (resistance of each set of conditions (25 h for the 0 s sample, 20 h for the 106 s R) and the imaginary part of the impedance (reactance X) are pre- sample, 15 h for the 130 s sample, and 14 h for the 186 s sample), sented using a Cole–Cole plot (Cole 1932). For biological tissues, in this study, the drying periods were regarded as being within the the Cole–Cole plot is described as a circular arc. Additionally, the first falling rate period. The drying constant k of each blanched sam- arc is shrunken by steam heating (Watanabe et al., 2017). Therefore, ple was larger than that of non-blanched sample (Table  1). It was this study focused on the coordinate at the top of the circular arc shown that the drying rate was increased by the blanching treatment. and used that to evaluate the CMS of samples after blanching as described by Watanabe et  al. (2016). The ratio of the distance between the coordinate at the top of the circular arc (CR) and the origin was calculated using Equation (4): CC − CR = (4) CC − 0 e where C is the distance between the coordinate at the top of the cir- cular arc and the origin of the sample (kΩ), C is the distance between the coordinate at the top of the circular arc and the origin of the heated sample after blanching treatment (kΩ), and C is the distance between the coordinate at the top of the circular arc and the origin of the fresh sample (kΩ). The distance between the coordinate at the top of the circular arc of a stable sample and the origin, and that of a damaged sample and the origin are expressed as 1 and 0, respectively. Energy consumption Figure  2. Changes in the moisture content ratio of paprika sample with or The energy consumption in the drying of each blanched sample and without blanching treatment during far-infrared (FIR) drying. Symbols in the non-blanched sample was calculated according to the following graph show the steam branching time. Dashed lines show the calculated equation: moisture content by Equation (1). Downloaded from https://academic.oup.com/fqs/article-abstract/2/2/97/4955764 by Ed 'DeepDyve' Gillespie user on 26 June 2018 100 T. Orikasa et al., 2018, Vol. 2, No. 2 Therefore, we calculated the drying rate of each sample in the first surfaces after blanching treatment, and they were found to be lower falling rate period. A characteristic drying curve is shown in Figure 3. than those of the non-blanched samples. The difference between the The drying rate decreased linearly after first few hours of drying. The hardness of samples blanched for 186  s and that of non-blanched average drying rates of each blanched sample in the first falling rate samples was significant ( P < 0.05), though the differences in hard- period were calculated to determine the effect of blanching time on ness between the various blanching conditions were not signifi - the drying rate. The average drying rate of samples blanched for 0 s, cant. Figure 5A shows the internal temperatures of samples during −1 −1 −1 −1 106 s, 130 s, and 186 s were 0.39 h , 0.47 h , 0.62 h , and 0.64 h , blanching treatment. The internal temperature reached 80°C within respectively. The results clearly show that the blanching treatment 70  s during the blanching process, and the temperature remained increased the drying rate of paprika in the first falling rate period. The drying rates of the samples blanched for 130 and 186 s were 1.6 times higher than that of the non-blanched samples. The causes of the increase in drying rate of the blanched samples are considered to be 1. increasing penetration of water into the sample surface by inhibiting the hardening of the sample surface (Orikasa et al., 2008), 2.  destroying CMS (Watanabe et  al., 2017), and 3.  changing the resistance to internal moisture diffusion by altering the microstruc- ture due to physical damage to the sample (Watanabe et al., 2014). The hardness of the sample surface, microstructure, and CMS were measured to investigate the effect of enhancing water penetration into the sample surface and resistance to internal moisture diffusion on increased drying time, and the factors responsible for increased drying rate are discussed in the following sections. Sample hardness Figure  4. Hardness of the sample surface of blanched samples and dried Orikasa et al. (2008) reported that hardening of sample surfaces dur- samples with a moisture content of 8.0 (d.b.). Data are the mean ± SD (n = 3). Different lower case letters indicate significant differences ( P  <  0.05) by ing drying inhibits facile moisture movement within a dried sample. Tukey–Kramer test. The horizontal axis shows the steam blanching time. This study also demonstrated that the hardening of the surface of the paprika samples noticeably decreased the drying rate. Therefore, the hardness of the sample surfaces before and after blanching treatment and during the drying process was measured to determine the effect of preventing hardening of the sample surfaces by blanching treat- ment on the drying rate. Figure 4 shows the hardness of the sample Table 1. Effects of blanching time on the drying constant of paprika in Equation (1) −1 2 Drying constant k (h ) RMSE (−) R 0 s 0.085 0.260 0.999 106 s 0.105 0.263 0.999 130 s 0.142 0.276 0.998 186 s 0.142 0.265 0.998 0, 106, 130, and 186 s show the steam branching time. Figure 5. Changes in paprika sample internal temperature during blanching Figure  3. Drying characteristic curves of paprika sample with or without and drying treatment. (A) is the sample temperature during blanching blanching treatment during far-infrared (FIR) drying. Symbols in the graph treatment. (B) is the sample temperature during the drying process. Symbols show the steam branching time. in the graph show the differences in steam branching time. Downloaded from https://academic.oup.com/fqs/article-abstract/2/2/97/4955764 by Ed 'DeepDyve' Gillespie user on 26 June 2018 Drying Rate and Energy Consumption During FIR Drying of Paprika, 2018, Vol. 2, No. 2 101 high until blanching was complete. These results suggested that the sample surface was softened by superheated steam blanching, which was caused by decomposition of the plant tissue into low-molecular- weight pectins by β-elimination during heating (Sila et al., 2009). Figure  4 shows the hardness of the sample surfaces during the drying process. The slight increase in the hardness under each set of conditions was confirmed; all samples were hardened by drying of sample surface. The hardness of the blanched samples was lower than those of the non-blanched samples. The differences of the hard- ness between the samples blanched for 130 and 186 s and that of the non-blanched samples were significant ( P < 0.05). The results clearly show that blanching pretreatment prevents hardening of the sam- ple surfaces during drying because the β-elimination from blanch- ing at high temperatures (over 80°C) causes the surfaces to soften. Inactivation of pectin methyl esterase (PME) is thought to be the other factor preventing hardening of sample surfaces during drying. Fuchigami et al. (1995) reported that the pectin network is strength- ened and the firmness of the tissue is maintained in further thermal processing when PME is activated. Sila et al. (2007) reported that the optimum operating temperature for PME is approximately 50°C. However, in this study, the hardness of the blanched samples during drying was almost the same as the hardness of the sample imme- diately after blanching treatment even though the temperature of the sample during drying was maintained at approximately 50°C (Figure  5B). These results show the PME was inactivated by the blanching pretreatment, and PME-related hardening did not occur during drying of the blanched sample. As shown in Figure  5A, the internal temperature of the sample reached to 80°C after 80  s of the blanching treatment. This temperature condition inactivates the PME activity (Ando et  al., 2016). Therefore, we estimated that the Figure  6. Cole–Cole plot of paprika samples after blanching treatment and steam blanching process decreased the hardness of the sample sur- during the drying process. (A) is the Cole–Cole plot of blanched samples. (B) face and PME activity, and then, the softening of the blanched sam- is the Cole–Cole plot of dried samples with a moisture content of 8.0 (d.b.). ples resulted in the increased in the drying rate. Symbols in the graph show the steam branching time. Cell membrane stability increased, and this trend was the same as that of the blanching treat- Figure 6A shows the Cole–Cole plot after each blanching treatment. ment as shown in Figure 7A. Ando et al. (2014) reported that drying In Figure 6A, the circular arc of the Cole–Cole plot of the samples and heat stresses led to injury of the cell membranes of potato sam- decreased as the duration of the heat treatment increased. Damage ples and resulted in a reduction of the circular arc in the Cole–Cole to the cell membrane occurs at temperatures above 40–50°C (Zhang plot. The same trends seen in previous studies were also found in this et al., 1993). The temperature of the sample during the blanching pro- study. Table 2 shows the CR values of each sample after blanching cess quickly reached approximately 100°C (Figure  5A). Therefore, treatment and during the drying process. The CR during the drying it is likely that the cell membranes of the blanched samples in this process of the non-blanched sample was 0.68, which was the same study were damaged by heating. The water permeability of the cell as the CR of the sample blanched for 106 s immediately following membranes was increased by the damage caused by heating (Ando blanching treatment (0.70). The CRs during the drying process of the et al., 2016). The drying rate would be improved by higher cell mem- other blanched samples were lower than that of the non-blanched brane water permeabilities because of the facile movement of water samples. These results suggested that the supply of water at the sur- from internal tissue to the sample surface. To determine the amount face of non-blanched samples might not be sufficient in the early of damage to the cell membrane during blanching and drying, we stages of drying because of the amount of time it takes for the water evaluated the CMS by using the ratio of the distance to the coordin- permeability of the internal tissue to increase. On the other hand, ate at the top of the circular arc (CR) following the method proposed the blanched samples had sufficient permeabilities of their internal by Watanabe et al. (2016). Figure 7A shows the coordinate at the top tissue from the blanching pretreatment, which allowed the internal of the circular arc of the sample after blanching treatment, and this water to quickly reach the sample surface. This phenomenon would coordinate moved closer to the origin as blanching time increased. contribute to the higher drying rate of the blanched samples. We This result shows that damage to the cell membrane increased as focused on the relationship between the CR in the drying treatment blanching time increased. The CR of the samples after 106 s, 130 s, and the average drying rate. The CRs in the drying process of each and 186 s of blanching treatment were 0.70, 0.61, and 0.22, respect- blanched sample were lower than that of non-blanched sample, and −1 −1 ively (Table 2). This result confirmed that blanching time impacts the the results of average drying rate (0.39 h for 0 s, 0.47 h for 106 s, −1 −1 extent of damage to the cell membranes. 0.62 h for 130 s, and 0.64 h for 186 s) showed the same trend. Figure  7B shows the coordinate at the top of the circular arc The results suggested that the enhancement of the water permeabil- of the sample during the drying process. The coordinate at the top ity of the internal tissue due to heat stress-related cell membrane of the circular arc moved closer to the origin as the blanching time damage increased the drying rate. Downloaded from https://academic.oup.com/fqs/article-abstract/2/2/97/4955764 by Ed 'DeepDyve' Gillespie user on 26 June 2018 102 T. Orikasa et al., 2018, Vol. 2, No. 2 Table 3. Effects of blanching time on energy consumption of dried paprika production process 0 s 106 s 130 s 186 s Energy consumption during 0 3.45 3.60 3.69 blanching treatment (kWh/kg) Energy consumption during drying 42.02 35.30 26.41 25.73 treatment (kWh/kg) Total energy consumption 42.02 38.75 30.02 29.42 (kWh/kg) 0, 106, 130, and 186 s show the steam branching time. process was 18–39% lower than that of the non-blanched sam- ples even though the energy consumption of each blanched sample during the blanching process was 3.45–3.69 kWh/kg. The energy consumption of the blanched samples during drying was consider- ably lower than that of the non-blanched samples. The total energy consumption of each blanched sample was also lower than that of non-blanched samples. The reductions in the total energy consump- tion for samples blanched for 106 s, 130 s, and 186 s were 10.2%, 31.3%, and 30.4%, respectively. Considerable energy savings were found for the samples blanched for 130 and 186 s because the dry- ing rates of these samples were 1.6 times larger than that of the non-blanched sample. These results clearly showed blanching pre- treatment considerably reduced the total energy consumption of the drying process. On a commercial scale, the relative energy costs of drying processes can vary greatly depending on the design of the apparatus and the operating parameters such as air flow, tempera - ture, and sample and chamber volume (Durance and Wang, 2002). Further research regarding the energy consumption of practical Figure  7. Coordinates at the top of the circular arc of the Cole–Cole plot of equipment for each step in the drying process is needed because the samples. (A) is the coordinate at the top of the circular arc of blanched samples. (B) is the coordinate at the top of the circular arc of dried samples large-scale and continuously operating equipment are expected to with a moisture content of 8.0 (d.b.). Data are the mean ± SD (n  =  6–8). be more efficient. Symbols in the graph show the steam branching time. Table 2. Effects of blanching time on CR values of paprika sample Conclusions after each treatment We discussed the effect of superheated steam blanching pretreatment CR after blanching CR of dried samples with a on the drying rate and energy consumption of paprika during FIR dry- moisture content of 8.0 (d.b.) ing. The average drying rates of paprika samples blanched for 130 s and −1 −1 186 s during FIR drying were 0.62 h and 0.64 h , respectively, and 0 s 1.00 0.60 these drying rates were 1.6 times faster than that of the non-blanched 106 s 0.79 0.55 samples. The hardness of the samples after blanching treatment and 130 s 0.55 0.45 during the drying process was lower than that of the non-blanched 186 s 0.28 0.17 samples. Preventing hardening of the sample surfaces during drying by blanching pretreatment increased the drying rate of the paprika. CR shows the ratio of the distance between the coordinate at the top of the The CRs, which is related to cell membrane integrity of the blanched circular arc. 0, 106, 130, and 186 s show the steam branching time. samples, decreased as the blanching time increased, and the damage to the cell membranes due to heat stress increased. Damage to the cell To summarize the results on the hardness of sample surface and cell membrane increases the drying rate because the CRs of dried samples membrane injury measurements, preventing hardening of the sample that had been blanched were lower than those of non-blanched sam- surfaces by β-elimination and injuring the cell membranes by heating ples. The mechanism by which the drying rate increased is believed to increased the drying rates of the paprika samples. The mechanism for be a combination of two factors: enhancing water permeability of the increasing the drying rate involves two factors: water in the cells leaked sample surface by preventing hardening of the sample surface during into the extracellular space due to changes in the water permeability the drying process, and enhancing water permeability of internal tis- of the cell membranes due to heat stress, and the water moved to the sue by preventing hardening accelerates water evaporation from sam- sample surface by capillary action. In addition, enhancing the water ple surface. The total energy consumption of the FIR drying process permeability of the sample surface by preventing hardening during the of paprika was reduced by approximately 30% by the introduction drying process accelerated water evaporation from the sample surface. of a blanching pretreatment step before drying. The effect was mainly Energy consumption caused by the 1.6 times increase in the drying rate compared with the The energy consumed during each process is shown in Table 3. The drying rate without blanching pretreatment. These findings contribute energy consumption of each blanched sample during the drying the development of environmentally friendly FIR drying techniques Downloaded from https://academic.oup.com/fqs/article-abstract/2/2/97/4955764 by Ed 'DeepDyve' Gillespie user on 26 June 2018 Drying Rate and Energy Consumption During FIR Drying of Paprika, 2018, Vol. 2, No. 2 103 Okamoto, S., et al. (2012). Application of far-infrared for drying of Komatsuna. for paprika and CO emission abatement for the production of pro- Journal of the Japanese Society for Food Science and Technology, 59: 465– cessed agricultural products. 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Journal of the Japanese Society tion rate of vacuum microwave and hot air convection dehydrated toma- of Agricultural Machinery and Food Engineers, 76: 387–394 (in Japanese toes. Journal of Food Science, 67: 2212–2216. with English abstract). Fuchigami, M., Miyazaki, K., Hyakumoto, N. (1995). Frozen carrots texture Watanabe, T., et al. (2015). Determination of optimum blanching conditions and pectic components as affected by low-temperature-blanching and to produce dried paprika by conjoint analysis. Journal of the Japanese quick freezing. Journal of Food Science, 60: 132–136. Society for Food Science and Technology, 62: 394–401 (in Japanese with Guerrero, L. G., Gálvez, A. P., Aranda, E., Mosquera, M. I. M., Méndes, D. H. English abstract). (2010). Physicochemical and microbiological characterization of the dehy- Watanabe, T., et  al. (2016). The influence of inhibit avoid water defect dration processing of red pepper fruits for paprika production. 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Thin-layer ing. Journal of the Japanese Society for Food Science and Technology, 61: drying characteristics and modeling of pistachio nuts. Journal of Food 151–159 (in Japanese with English abstract). Engineering, 78: 98–108. Zhang, M. I.  N., Willison, J. H.  M., Cox, M. A., Hall, S. A. (1993). Nilnakara, S., Chiewchan, N., Devahastin, S. (2009). Production of anti- Measurement of heat injury in plant tissue by using electrical impedance oxidant dietary fibre powder from cabbage outer leaves. Food and analysis. Canadian Journal of Botany, 71: 1605–1611. Bioproducts Processing, 87: 301–307. Downloaded from https://academic.oup.com/fqs/article-abstract/2/2/97/4955764 by Ed 'DeepDyve' Gillespie user on 26 June 2018 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Food Quality and Safety Oxford University Press

Impact of blanching pretreatment on the drying rate and energy consumption during far-infrared drying of Paprika (Capsicum annuum L.)

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Oxford University Press
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© The Author(s) 2018. Published by Oxford University Press on behalf of Zhejiang University Press.
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2399-1399
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2399-1402
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10.1093/fqsafe/fyy006
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Abstract

We incorporated a superheated steam blanching pretreatment step into a paprika drying process and compared the far-infrared (FIR) drying rates, hardness of the sample surfaces, cell membrane stabilities, and energy consumption of blanched and non-blanched paprika. The average drying rate of blanched paprika samples during FIR drying was higher than that of non-blanched samples. The hardness and cell membrane stability of dried blanched samples were lower than those of non-blanched samples. We estimated that the softening of the sample surfaces and injury to the cell membranes caused the drying rate to increase. The total energy consumption of the FIR drying of paprika was reduced by approximately 30% by introducing the blanching pretreatment. These findings contribute to the development of environmentally friendly FIR drying techniques for paprika. Key words: blanching; cell membrane stability, energy consumption; far-infrared drying; hardness. changes in plant materials that affect the quality of final products, Introduction such as excessive shrinkage, discoloration, oxidation of functional Dried paprika powder is commonly used throughout the world. ingredients, and severe deterioration of nutritional and sensorial Paprika powder has features such as a high nutritional content properties. Far-infrared (FIR) drying can be used as an alternative (including L-ascorbic acid and polyphenol) and good clear colour; to hot-air drying. FIR drying has remarkable advantages including therefore, it is widely used as a spice for cooking. Previous stud- a shorter drying time and improved energy efficiency ( Hebbar et al., ies have reported the drying characteristics and quality changes of 2004). Okamoto et  al. (2012) reported that the energy consump- paprika during the drying process in order to prepare high-quality tion of FIR drying for Japanese mustard spinach leaves was approxi- dried paprika (Topuz et  al., 2009; Guerrero et  al., 2010; Ramesh mately 17% lower than that of hot-air drying. Development of novel et al., 2001). techniques for more environmentally friendly drying methods would Hot-air drying is a simple method commonly used for fruits and contribute to CO emission abatement in the processing of agricul- vegetables (Bazyma et al., 2006). Some disadvantages of hot-air dry- tural products. Increased use of more environmentally friendly dry- ing regarding the drying process parameters include low energy effi - ing techniques over conventional techniques would have advantages ciency and the lengthy drying times required during the falling rate not only towards CO emission abatement but also reduced running period (Drouzas et al., 1999; Bazyma et al., 2006). Traditional dry- costs and improved productivity. ing methods, including hot-air drying, can cause many undesirable © The Author(s) 2018. Published by Oxford University Press on behalf of Zhejiang University Press. This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact journals.permissions@oup.com Downloaded from https://academic.oup.com/fqs/article-abstract/2/2/97/4955764 by Ed 'DeepDyve' Gillespie user on 26 June 2018 98 T. Orikasa et al., 2018, Vol. 2, No. 2 Increasing the drying rate would help reduce energy consumption were cooled immediately in ice water (0°C for 150  s), and then during the drying process. As expected, we found that the blanching excess water on the sample surface was absorbed by paper towel. treatment could enhance the drying rate. Generally, blanching treat- After cooling, the blanched samples were dried using a FIR drying ment can inhibit various undesirable enzymatic reactions and thus chamber. may be required to improve the final quality of the processed prod - uct (Nilnakara et  al., 2009). In addition, blanching pretreatments FIR drying treatment naturally enhance the drying rates of kiwifruit (Yoshida et al., 2014), The drying experiments were conducted in a FIR drying apparatus spinach (Hirota et al., 2000), and cabbage (Nilnakara et al., 2009; as illustrated in Figure  1. The system consists of a drying chamber Watanabe et  al., 2014). Watanabe et  al. (2015) evaluated quality (DKM600, Yamato, Japan) with two FIR heating panels (TE Super changes in paprika slices during an FIR drying process; however, aguri-heater, Toyo Kosan, Japan) connected to 100 V AC power the effects of blanching pretreatment on the drying rate of paprika source. The air velocity during drying was 0.3 m/s, which was meas- samples have not been reported. Therefore, the first objective of this ured by an anemometer (405-V1, Testo, Japan). Six samples were study is to quantitatively evaluate the effects of blanching pretreat- placed skin side down on the steel net at a height of 26 cm in the ment on the drying rate of blanched paprika samples. drying chamber. The sample was considered to have reached its Several reasons why blanching pretreatment accelerates drying equilibrium moisture content when the mass change during 1.0 h of have been proposed. These reasons are as follows: the water perme- drying was less than 0.01 g, and once equilibrium was reached, the ability of the surface is enhanced due to decreased hardening of the drying test was terminated. The mass of the sample during drying sample surface (Orikasa et  al., 2008), internal resistance to mois- was measured at 1.0  h intervals by retrieving the sample from the ture diffusion is reduced by changes in the microstructure due to drying chamber and weighing it using a digital balance (GF-3000, physical damage to the sample (Watanabe et al., 2014), and internal A&D, Japan). After the measurement, the sample was returned to its water permeability is improved due to damage to the cell membranes original position, and the drying experiment was continued. by heat stress (Ando et  al., 2016). These factors interact in a com- plicated manner to enhance the drying rate. However, these previ- Modelling of moisture content changes ous studies have not evaluated the effects and mechanism in detail. Many studies have demonstrated that the changes in the moisture Therefore, the second objective of this study is to evaluate the effects content of agricultural products during the falling rate period of of the blanching treatment that are responsible for the enhanced dry- their drying processes can be predicted by the following exponential ing rate during FIR drying of paprika. This was achieved by meas- model (Kashaninejad et  al., 2007; Srikiatdena and Roberts, 2008; uring the sample surface hardness after blanching and during the Orikasa et al., 2014): drying process, and cell membrane stability (CMS), which was used MM − as an indicator of physical damage of samples. MR = =− exp( kt) (1) MM − 0 e In addition, Ando et  al. (2016) mentioned the importance of investigating the effects of increased drying rate due to blanch- ing treatment on the total energy consumption of the drying pro- where M is the moisture content (d.b.), M is the equilibrium mois- cess. Even if energy consumption during the drying process can be ture content (d.b.), M is the initial moisture content (d.b.), and k is reduced by introducing blanching pretreatment to the paprika dry- the proportional constant (called the drying rate constant) in the first −1 ing process, total energy consumption might be increased due to the drying falling rate period (h ). The M in this study was regarded as energy consumption of the blanching process, and this additional the final moisture content at the end of the drying period [0.0111 step might not contribute to overall CO emission abatement of (d.b.) for the sample blanched for 0 s, 0.1939 (d.b.) for the sample the total process. Therefore, the third objective of this study is to blanched for 103 s, 0.2030 (d.b.) for the sample blanched for 130 s, evaluate the effect of increase of drying rate by the incorporation of and 0.0050 (d.b.) for the sample blanched for 186 s]. The drying blanching treatment on the total energy consumption of the dried constant k was estimated by fitting the obtained moisture content paprika production process. data to Equation (1) using the least squares method. To evaluate the goodness of the fit, the root mean squared error (RMSE) and the determination coefficient ( R ) were calculated. Materials and Methods Materials Paprika (Capsicum annuum L.) was purchased from a local market and stored in a refrigerator at 4°C before measurements. The pap- rika was analysed within 1 week. Once the stalk and seeds were removed, the paprika was divided into eight pieces using a kitchen knife. The mass of each piece was approximately 15  g. The initial moisture content of these samples was 11.35  ±  0.004 [kg-water/ kg-dry solid (d.b.)] as determined using the oven drying method reported by Topuz et al. (2009). Blanching treatment The blanching treatments were conducted using a superheated steam oven (AX-PX-W, SHARP, Japan) at 200°C in a chamber in preheat- ing mode. Six samples were placed skin side down on the steel net in the steam oven chamber. The samples were heated for 0, 106, Figure 1. Schematic diagram of the far-infrared (IFR) drying apparatus. 130, and 186 s (Watanabe et  al., 2015). After heating, the samples Downloaded from https://academic.oup.com/fqs/article-abstract/2/2/97/4955764 by Ed 'DeepDyve' Gillespie user on 26 June 2018 Drying Rate and Energy Consumption During FIR Drying of Paprika, 2018, Vol. 2, No. 2 99 Hardness of the sample surface QP =× ()tW () −W (5) 0e Punching tests were performed to evaluate changes in sample hard- where Q is the energy consumption per 1 kg of water removed via ness after blanching treatment and during the drying process. The the drying process (kJ/kg), P is the electrical power supplied during tests were performed using a food hardness meter (RE2-3305S, the blanching and drying processes (kW), t is the treatment time (h), YAMADEN, Japan) with a 20 N load cell and a flat-ended cylindrical W is the initial mass of the sample (kg), and W is the mass of the probe 3 mm in diameter. The displacement speed was 10 mm/s. The 0 e dried sample at a moisture content of 0.1 (d.b.) (kg). The energy sample was placed on a polyacetal resin flat plate in a food hardness consumptions of the blanching and drying processes were measured meter. In this study, the average fracture load at the centre point of using a power meter (CW240, Yokogawa Meters & Instruments six samples was defined as the hardness of the sample. The blanched Corporation, Japan). The power meter was connected to the power samples were equilibrated at room temperature for 10 min before the source, and the drying devices (i.e., the steam oven and drying cham- hardness measurements because the effect of increased turgor pressure ber) were connected to the power meter. The apparent electric energy in the cells during the blanching process on the hardness of the sam- consumptions during operation of the drying devices were recorded ples (Tamura 1995) must be excluded. The hardness of the samples by the power meter in kWh and were assumed to be the energy con- during the drying process was measured when the samples reached sumptions of the respective processes. a moisture content of 8.0 (d.b.). The measurements for each sample under each set of conditions were conducted in triplicate. Statistics Quantitative data are presented as the means ± the SE. For the stat- Cell membrane stability istical analyses, Tukey–Kramer tests with a significance level of The CMS of fresh and blanched paprika samples was determined P  <  0.05 were applied by using statistics software (BellCurve for by electrical impedance spectroscopy (EIS) as described by Wu et al. Excel, ver. 2.13, Social Survey Research Information Co., Ltd). (2008). The impedance [impedance magnitude |Z| (Ω) and phase dif- ference θ (rad)] of the samples was measured using an LCR tester Results and Discussion with two parallel electrodes spaced 10  mm apart at 50 frequency points (logarithmic frequency intervals) over the frequency range of Drying model and drying rate 42 Hz to 5 MHz at a measuring voltage of 1.0 V (frequency accur- Previous studies have reported that changes in the moisture content acy: < ±0.005%; measurable impedance range: 10.00 mΩ to 200.00 of fruits and vegetables during drying processes that removed more MΩ; HIOKI, 3532-50, Japan), and the values were automatically than approximately 0.1 of the moisture content ratio could be pre- recorded by a computer for analysis. The resistance R and reactance dicted by the exponential model (Ayensu 1997; Sacilik and Elicin, X were calculated from the following equations: 2006; Doymaz, 2007), and the model showed that drying primar- ily occurred in the first falling rate period. To evaluate the drying RZ = cosθ (2) periods in this study, we fitted the moisture content data from the initial measurement to a moisture content ratio of 0.1 to Equation (1). The solid lines in Figure 2 represent the results calculated from XZ = sinθ (3) Equation (1). The calculated results agreed with the measured results (RMSE = 0.260–0.276; R  = 0.998–0.999). Within the drying time The relationships between the real part of the impedance (resistance of each set of conditions (25 h for the 0 s sample, 20 h for the 106 s R) and the imaginary part of the impedance (reactance X) are pre- sample, 15 h for the 130 s sample, and 14 h for the 186 s sample), sented using a Cole–Cole plot (Cole 1932). For biological tissues, in this study, the drying periods were regarded as being within the the Cole–Cole plot is described as a circular arc. Additionally, the first falling rate period. The drying constant k of each blanched sam- arc is shrunken by steam heating (Watanabe et al., 2017). Therefore, ple was larger than that of non-blanched sample (Table  1). It was this study focused on the coordinate at the top of the circular arc shown that the drying rate was increased by the blanching treatment. and used that to evaluate the CMS of samples after blanching as described by Watanabe et  al. (2016). The ratio of the distance between the coordinate at the top of the circular arc (CR) and the origin was calculated using Equation (4): CC − CR = (4) CC − 0 e where C is the distance between the coordinate at the top of the cir- cular arc and the origin of the sample (kΩ), C is the distance between the coordinate at the top of the circular arc and the origin of the heated sample after blanching treatment (kΩ), and C is the distance between the coordinate at the top of the circular arc and the origin of the fresh sample (kΩ). The distance between the coordinate at the top of the circular arc of a stable sample and the origin, and that of a damaged sample and the origin are expressed as 1 and 0, respectively. Energy consumption Figure  2. Changes in the moisture content ratio of paprika sample with or The energy consumption in the drying of each blanched sample and without blanching treatment during far-infrared (FIR) drying. Symbols in the non-blanched sample was calculated according to the following graph show the steam branching time. Dashed lines show the calculated equation: moisture content by Equation (1). Downloaded from https://academic.oup.com/fqs/article-abstract/2/2/97/4955764 by Ed 'DeepDyve' Gillespie user on 26 June 2018 100 T. Orikasa et al., 2018, Vol. 2, No. 2 Therefore, we calculated the drying rate of each sample in the first surfaces after blanching treatment, and they were found to be lower falling rate period. A characteristic drying curve is shown in Figure 3. than those of the non-blanched samples. The difference between the The drying rate decreased linearly after first few hours of drying. The hardness of samples blanched for 186  s and that of non-blanched average drying rates of each blanched sample in the first falling rate samples was significant ( P < 0.05), though the differences in hard- period were calculated to determine the effect of blanching time on ness between the various blanching conditions were not signifi - the drying rate. The average drying rate of samples blanched for 0 s, cant. Figure 5A shows the internal temperatures of samples during −1 −1 −1 −1 106 s, 130 s, and 186 s were 0.39 h , 0.47 h , 0.62 h , and 0.64 h , blanching treatment. The internal temperature reached 80°C within respectively. The results clearly show that the blanching treatment 70  s during the blanching process, and the temperature remained increased the drying rate of paprika in the first falling rate period. The drying rates of the samples blanched for 130 and 186 s were 1.6 times higher than that of the non-blanched samples. The causes of the increase in drying rate of the blanched samples are considered to be 1. increasing penetration of water into the sample surface by inhibiting the hardening of the sample surface (Orikasa et al., 2008), 2.  destroying CMS (Watanabe et  al., 2017), and 3.  changing the resistance to internal moisture diffusion by altering the microstruc- ture due to physical damage to the sample (Watanabe et al., 2014). The hardness of the sample surface, microstructure, and CMS were measured to investigate the effect of enhancing water penetration into the sample surface and resistance to internal moisture diffusion on increased drying time, and the factors responsible for increased drying rate are discussed in the following sections. Sample hardness Figure  4. Hardness of the sample surface of blanched samples and dried Orikasa et al. (2008) reported that hardening of sample surfaces dur- samples with a moisture content of 8.0 (d.b.). Data are the mean ± SD (n = 3). Different lower case letters indicate significant differences ( P  <  0.05) by ing drying inhibits facile moisture movement within a dried sample. Tukey–Kramer test. The horizontal axis shows the steam blanching time. This study also demonstrated that the hardening of the surface of the paprika samples noticeably decreased the drying rate. Therefore, the hardness of the sample surfaces before and after blanching treatment and during the drying process was measured to determine the effect of preventing hardening of the sample surfaces by blanching treat- ment on the drying rate. Figure 4 shows the hardness of the sample Table 1. Effects of blanching time on the drying constant of paprika in Equation (1) −1 2 Drying constant k (h ) RMSE (−) R 0 s 0.085 0.260 0.999 106 s 0.105 0.263 0.999 130 s 0.142 0.276 0.998 186 s 0.142 0.265 0.998 0, 106, 130, and 186 s show the steam branching time. Figure 5. Changes in paprika sample internal temperature during blanching Figure  3. Drying characteristic curves of paprika sample with or without and drying treatment. (A) is the sample temperature during blanching blanching treatment during far-infrared (FIR) drying. Symbols in the graph treatment. (B) is the sample temperature during the drying process. Symbols show the steam branching time. in the graph show the differences in steam branching time. Downloaded from https://academic.oup.com/fqs/article-abstract/2/2/97/4955764 by Ed 'DeepDyve' Gillespie user on 26 June 2018 Drying Rate and Energy Consumption During FIR Drying of Paprika, 2018, Vol. 2, No. 2 101 high until blanching was complete. These results suggested that the sample surface was softened by superheated steam blanching, which was caused by decomposition of the plant tissue into low-molecular- weight pectins by β-elimination during heating (Sila et al., 2009). Figure  4 shows the hardness of the sample surfaces during the drying process. The slight increase in the hardness under each set of conditions was confirmed; all samples were hardened by drying of sample surface. The hardness of the blanched samples was lower than those of the non-blanched samples. The differences of the hard- ness between the samples blanched for 130 and 186 s and that of the non-blanched samples were significant ( P < 0.05). The results clearly show that blanching pretreatment prevents hardening of the sam- ple surfaces during drying because the β-elimination from blanch- ing at high temperatures (over 80°C) causes the surfaces to soften. Inactivation of pectin methyl esterase (PME) is thought to be the other factor preventing hardening of sample surfaces during drying. Fuchigami et al. (1995) reported that the pectin network is strength- ened and the firmness of the tissue is maintained in further thermal processing when PME is activated. Sila et al. (2007) reported that the optimum operating temperature for PME is approximately 50°C. However, in this study, the hardness of the blanched samples during drying was almost the same as the hardness of the sample imme- diately after blanching treatment even though the temperature of the sample during drying was maintained at approximately 50°C (Figure  5B). These results show the PME was inactivated by the blanching pretreatment, and PME-related hardening did not occur during drying of the blanched sample. As shown in Figure  5A, the internal temperature of the sample reached to 80°C after 80  s of the blanching treatment. This temperature condition inactivates the PME activity (Ando et  al., 2016). Therefore, we estimated that the Figure  6. Cole–Cole plot of paprika samples after blanching treatment and steam blanching process decreased the hardness of the sample sur- during the drying process. (A) is the Cole–Cole plot of blanched samples. (B) face and PME activity, and then, the softening of the blanched sam- is the Cole–Cole plot of dried samples with a moisture content of 8.0 (d.b.). ples resulted in the increased in the drying rate. Symbols in the graph show the steam branching time. Cell membrane stability increased, and this trend was the same as that of the blanching treat- Figure 6A shows the Cole–Cole plot after each blanching treatment. ment as shown in Figure 7A. Ando et al. (2014) reported that drying In Figure 6A, the circular arc of the Cole–Cole plot of the samples and heat stresses led to injury of the cell membranes of potato sam- decreased as the duration of the heat treatment increased. Damage ples and resulted in a reduction of the circular arc in the Cole–Cole to the cell membrane occurs at temperatures above 40–50°C (Zhang plot. The same trends seen in previous studies were also found in this et al., 1993). The temperature of the sample during the blanching pro- study. Table 2 shows the CR values of each sample after blanching cess quickly reached approximately 100°C (Figure  5A). Therefore, treatment and during the drying process. The CR during the drying it is likely that the cell membranes of the blanched samples in this process of the non-blanched sample was 0.68, which was the same study were damaged by heating. The water permeability of the cell as the CR of the sample blanched for 106 s immediately following membranes was increased by the damage caused by heating (Ando blanching treatment (0.70). The CRs during the drying process of the et al., 2016). The drying rate would be improved by higher cell mem- other blanched samples were lower than that of the non-blanched brane water permeabilities because of the facile movement of water samples. These results suggested that the supply of water at the sur- from internal tissue to the sample surface. To determine the amount face of non-blanched samples might not be sufficient in the early of damage to the cell membrane during blanching and drying, we stages of drying because of the amount of time it takes for the water evaluated the CMS by using the ratio of the distance to the coordin- permeability of the internal tissue to increase. On the other hand, ate at the top of the circular arc (CR) following the method proposed the blanched samples had sufficient permeabilities of their internal by Watanabe et al. (2016). Figure 7A shows the coordinate at the top tissue from the blanching pretreatment, which allowed the internal of the circular arc of the sample after blanching treatment, and this water to quickly reach the sample surface. This phenomenon would coordinate moved closer to the origin as blanching time increased. contribute to the higher drying rate of the blanched samples. We This result shows that damage to the cell membrane increased as focused on the relationship between the CR in the drying treatment blanching time increased. The CR of the samples after 106 s, 130 s, and the average drying rate. The CRs in the drying process of each and 186 s of blanching treatment were 0.70, 0.61, and 0.22, respect- blanched sample were lower than that of non-blanched sample, and −1 −1 ively (Table 2). This result confirmed that blanching time impacts the the results of average drying rate (0.39 h for 0 s, 0.47 h for 106 s, −1 −1 extent of damage to the cell membranes. 0.62 h for 130 s, and 0.64 h for 186 s) showed the same trend. Figure  7B shows the coordinate at the top of the circular arc The results suggested that the enhancement of the water permeabil- of the sample during the drying process. The coordinate at the top ity of the internal tissue due to heat stress-related cell membrane of the circular arc moved closer to the origin as the blanching time damage increased the drying rate. Downloaded from https://academic.oup.com/fqs/article-abstract/2/2/97/4955764 by Ed 'DeepDyve' Gillespie user on 26 June 2018 102 T. Orikasa et al., 2018, Vol. 2, No. 2 Table 3. Effects of blanching time on energy consumption of dried paprika production process 0 s 106 s 130 s 186 s Energy consumption during 0 3.45 3.60 3.69 blanching treatment (kWh/kg) Energy consumption during drying 42.02 35.30 26.41 25.73 treatment (kWh/kg) Total energy consumption 42.02 38.75 30.02 29.42 (kWh/kg) 0, 106, 130, and 186 s show the steam branching time. process was 18–39% lower than that of the non-blanched sam- ples even though the energy consumption of each blanched sample during the blanching process was 3.45–3.69 kWh/kg. The energy consumption of the blanched samples during drying was consider- ably lower than that of the non-blanched samples. The total energy consumption of each blanched sample was also lower than that of non-blanched samples. The reductions in the total energy consump- tion for samples blanched for 106 s, 130 s, and 186 s were 10.2%, 31.3%, and 30.4%, respectively. Considerable energy savings were found for the samples blanched for 130 and 186 s because the dry- ing rates of these samples were 1.6 times larger than that of the non-blanched sample. These results clearly showed blanching pre- treatment considerably reduced the total energy consumption of the drying process. On a commercial scale, the relative energy costs of drying processes can vary greatly depending on the design of the apparatus and the operating parameters such as air flow, tempera - ture, and sample and chamber volume (Durance and Wang, 2002). Further research regarding the energy consumption of practical Figure  7. Coordinates at the top of the circular arc of the Cole–Cole plot of equipment for each step in the drying process is needed because the samples. (A) is the coordinate at the top of the circular arc of blanched samples. (B) is the coordinate at the top of the circular arc of dried samples large-scale and continuously operating equipment are expected to with a moisture content of 8.0 (d.b.). Data are the mean ± SD (n  =  6–8). be more efficient. Symbols in the graph show the steam branching time. Table 2. Effects of blanching time on CR values of paprika sample Conclusions after each treatment We discussed the effect of superheated steam blanching pretreatment CR after blanching CR of dried samples with a on the drying rate and energy consumption of paprika during FIR dry- moisture content of 8.0 (d.b.) ing. The average drying rates of paprika samples blanched for 130 s and −1 −1 186 s during FIR drying were 0.62 h and 0.64 h , respectively, and 0 s 1.00 0.60 these drying rates were 1.6 times faster than that of the non-blanched 106 s 0.79 0.55 samples. The hardness of the samples after blanching treatment and 130 s 0.55 0.45 during the drying process was lower than that of the non-blanched 186 s 0.28 0.17 samples. Preventing hardening of the sample surfaces during drying by blanching pretreatment increased the drying rate of the paprika. CR shows the ratio of the distance between the coordinate at the top of the The CRs, which is related to cell membrane integrity of the blanched circular arc. 0, 106, 130, and 186 s show the steam branching time. samples, decreased as the blanching time increased, and the damage to the cell membranes due to heat stress increased. Damage to the cell To summarize the results on the hardness of sample surface and cell membrane increases the drying rate because the CRs of dried samples membrane injury measurements, preventing hardening of the sample that had been blanched were lower than those of non-blanched sam- surfaces by β-elimination and injuring the cell membranes by heating ples. The mechanism by which the drying rate increased is believed to increased the drying rates of the paprika samples. The mechanism for be a combination of two factors: enhancing water permeability of the increasing the drying rate involves two factors: water in the cells leaked sample surface by preventing hardening of the sample surface during into the extracellular space due to changes in the water permeability the drying process, and enhancing water permeability of internal tis- of the cell membranes due to heat stress, and the water moved to the sue by preventing hardening accelerates water evaporation from sam- sample surface by capillary action. In addition, enhancing the water ple surface. The total energy consumption of the FIR drying process permeability of the sample surface by preventing hardening during the of paprika was reduced by approximately 30% by the introduction drying process accelerated water evaporation from the sample surface. of a blanching pretreatment step before drying. The effect was mainly Energy consumption caused by the 1.6 times increase in the drying rate compared with the The energy consumed during each process is shown in Table 3. The drying rate without blanching pretreatment. These findings contribute energy consumption of each blanched sample during the drying the development of environmentally friendly FIR drying techniques Downloaded from https://academic.oup.com/fqs/article-abstract/2/2/97/4955764 by Ed 'DeepDyve' Gillespie user on 26 June 2018 Drying Rate and Energy Consumption During FIR Drying of Paprika, 2018, Vol. 2, No. 2 103 Okamoto, S., et al. (2012). Application of far-infrared for drying of Komatsuna. for paprika and CO emission abatement for the production of pro- Journal of the Japanese Society for Food Science and Technology, 59: 465– cessed agricultural products. 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Published: Mar 28, 2018

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